Fibre laser comprising a mode-selective cavity mirror

ABSTRACT

The invention relates to a laser comprising a cavity which is defined by an end mirror and a decoupling mirror. A fibre comprising an active core is arranged in said cavity and can be excited by pump radiation until it has a multimodal laser activity, in such a way that a plurality of transversal modes form in the cavity. A mixture of modes is present in the fibre, and the decoupling mirror comprises reflection properties for laser and pump radiation, said properties varying according to the location, such that it reflects the pump radiation and the laser radiation which does not leave the active core of the fibre, and significantly decouples low transversal modes.

The invention relates to a laser comprising a resonator, which islimited by an end mirror and an output mirror and in which a fibrecomprising an active core is arranged.

It is well-known from the literature to configure laser resonators suchthat a diffraction-limited light emission occurs. In all thesearrangements, a suitable design of the resonator ensures that radiationhaving high beam quality is sufficiently amplified in the resonator. Incontrast thereto, radiation having poor beam quality is suppressed byinternal losses or by out-of-phase superposition. It is also known touse unstable resonators, e.g. from S. Townsend, J. Reilly, Unobscuredunstable resonator design for large bore lasers, Proc. SPIE Vol. 0147,p. 184-188, 1989. Such fibre lasers have the disadvantage, however, thatthe beam quality of the pump radiation directly affects the beam qualityof the emitted laser radiation. Since laser activity is primarilystimulated in the fibre core, the radiation intensity that can becoupled in is limited at the same time by the brilliance of the sourceof radiation for a given numerical aperture of the fibre. As a rule, thediameter of the fibre core then determines the diameter of the emittedbeam. Double-core fibres offer a certain remedy here; however, they arecomplex and expensive in manufacture. Also, efficient coupling betweenthe inner and outer core requires a great fibre length, which leads notonly to increased constructional dimensions, but also to increasedlosses in the laser resonator due to inevitable scattering andabsorption.

Due to this correspondence between the beam quality of the pumpingsource and the beam quality of the radiation emitted by the fibre laser,the use of a fibre laser has hitherto been linked inevitably withrelatively complex pumping sources or has been limited by the power ofthe pumping source, respectively.

Therefore, it is an object of the invention to provide a fibre laserwhich is capable of generating laser radiation of high beam quality evenwhen using pump radiation of poor beam quality.

According to the invention, the object is achieved by a laser comprisinga resonator which is limited by an end mirror and an output mirror andin which a fibre is arranged that comprises an active core and can bestimulated by pump radiation to have multi-mode laser activity such thata plurality of transverse modes occur in the resonator, wherein modemixing occurs in the fibre and the output mirror has reflectionproperties for laser and pump radiation which reflection properties varylocally such that the output mirror reflects pump radiation as well aslaser radiation that does not exit from the active core of the fibre,and thus couples out low transverse modes predominantly.

Thus, according to the invention, a multi-mode field is deliberatelystimulated in a fibre laser. The output mirror has the function of amode stop which prefers a transmission of the basic mode, but largelysuppresses coupling-out of radiation of higher transverse modes. Suchhigher modes preferably remain within the fibre, because due to modemixing which always occurs in a fibre and can be optimized by means offibre design and further measures, a renewed coupling-in of power fromthe higher mode into the basic mode occurs.

The concept according to the invention allows the use of a largecoupling-in surface area for pump radiation without degrading the beamquality of the emitted laser radiation thereby. For example, monocorefibres having a very large core diameter can be used without the qualityof the emitted radiation changing automatically as the cross section ofthe active core increases.

In addition to the basic mode, higher transverse modes are also formed,wherein the output mirror of the invention causes a corresponding modeselection for the laser beam. The efficient mode mixing in multi-modefibres ensures that all propagable modes within the active medium areamplified and, thus, the inversion generated by a multi-mode pumpingsource can be efficiently used. Nevertheless, the coupling-out ofradiation of higher or adjustable beam quality, respectively, islimited, and the radiation of lower beam quality remains within theresonator.

Moreover, the brilliance of the pumping source now no longer representsa noticeable limit for the power of the laser system. With a highnumerical aperture of the fibre, a high pump radiation intensity can becoupled in without there being particularly high demands on thebrilliance of the pumping source. The invention thus avoids thebottleneck caused, in the lasers according to the prior art, by theclose link between the maximum input pump density (as a product ofintensity and cross sectional area) and the diameter of the emittedlaser beam. The diameter of the active core can now be selected to bemuch larger than the diameter of the emitted beam, so that the beamquality of the pump radiation can accordingly be inferior to that of theemitted laser radiation. In other words, this means that the beamquality of the emitted radiation in the laser according to the inventionis improved over that of the pump radiation. This feature, which isactually known only for expensive double-core fibres, is now achieved ina much simpler way and without the aforementioned disadvantages of thedouble-core principle.

Further, the concept of defining the beam diameter by means of theoutput mirror also allows the use of fibres comprising active coreswhich are not circular in cross section. Thus, for example, a D-shapedcross section may be employed for the active core, which is particularlygood for coupling different transverse modes with each other.

In a simple embodiment of the concept according to the invention, theoutput mirror comprises, and is preceded by, a mode stop having suitableproperties. In many cases, however, the laser efficiency decreases ifsaid mode stop does not reflect the pump radiation. It is thereforeadvantageous for this simple embodiment to provide the output mirror asa mode stop reflecting the pump radiation.

In a further embodiment of this simple design, the output mirrorcomprises an inner zone and an outer zone surrounding said inner zone,wherein said outer zone reflects laser radiation and pump radiation andsaid inner zone has a lower degree of reflectivity for laser radiationthan the outer zone. The locally varying reflecting property of theoutput mirror is then realized in the form of two differently reflectingzones. The form of the inner zone has an effect on the beam crosssection and will, thus, usually be selected depending on the particularapplication. Such an output mirror comprising an inner zone and an outerzone is relatively easy to manufacture, and may be manufactured, inparticular, by coating one end of the fibre. Such direct coating isadvantageous in view of the fact that no further separate adjustingsteps are then required.

For most applications, a laser beam having a circular beam diameter isdesired. The inner zone will then usually have a circular design. Forthis purpose, the inner zone is advantageously circular, having asmaller diameter than the diameter of the active core. The beam qualityis increased from the coupled-in pump radiation to the coupled-out laserradiation by the ratio by which the inner zone is smaller than the crosssectional area of the active core. In this case, a suitable design ofthe inner zone relative to the cross sectional area of the active coreallows selection of almost any desired ratio.

In view of easy manufacture it is preferred not to provide the outputmirror directly on the end of a fibre, but to realize it as a discreteelement, wherein beam-expanding optics may optionally be provided, too,between the end of the fibre and the output mirror. In order to achievethe amplified output of low transverse modes, i.e. in order to obtainlaser radiation that is as monomodal as possible, the cross section ofthe inner zone is, in this case, always smaller than the expanded crosssection of the active core. For a circular inner zone and a circularactive core it is then advantageous, for example, to provide the innerzone with a smaller diameter than the expanded diameter of the activecore.

A further possibility to shape the generated laser radiation in terms ofbeam profile, intensity distribution and propagation characteristicswith a view to the requirements of particular applications is to arrangethe inner zone such that it is not coaxial to the radiation exiting theactive core. This allows selective admixing of higher transverse modes,which has a direct effect on intensity distribution and, thus, on thebeam profile.

In the concept according to the invention, the radiation which has notbeen coupled out is reflected back again and remains in the resonator.Said radiation is, in particular, radiation of higher transverse modeswhich are stimulated due to the geomety of the resonator and, inparticular, in the case of a high numerical aperture of the active coreof the fibre in the resonator. The mixture of different transverse modesachieves a very uniform intensity distribution over the cross section ofthe inner zone of the output mirror. The energy of radiation from modes(which have not been coupled out) is reflected back again into theresonator and is ultimately introduced into the coupled-out lowtransverse modes by mode mixing which inherently occurs in the fibre. Inorder to promote said mode mixing, a layout of the fibre in loops orbends is advantageous.

A further possibility to amplify the internal mode mixing of the fibrein the laser according to the invention consists in using a fibre whoseactive core has a D-shaped cross section. In such fibres, themode-mixing properties are particularly pronounced. Therefore, they areespecially suitable for the laser according to the invention.

The properties of the output mirror define the beam cross section of thecoupled-out laser beam. In a resonator allowing stimulation of laseractivity at a plurality of wavelengths, the spectral reflectionproperties of the output mirror also have an effect on the spectralcomposition of the emitted laser beam. Thus, a suitable selection of thereflection properties or of the transmission properties, respectively,of the output mirror allows both the diameter and the wavelength of theoutput laser beam to be influenced. This makes it possible to easilyobtain an adjustable or switchable laser, respectively, by providing anexchangeable output mirror.

The invention will be explained in more detail below, by way of exampleand with reference to the Figures, wherein:

FIG. 1 shows a schematic drawing of a fibre laser;

FIG. 2 shows a schematic representation of an end surface of the fibreof the fibre laser with an output mirror provided thereon;

FIG. 3 shows a top view of the end surface of FIG. 2;

FIG. 4 shows a schematic representation of a further output miror;

FIG. 5 shows a graph illustrating the beam profile, and

FIG. 6 shows a section through the end surface of a fibre of a laser ina graph illustrating the intensity of one mode.

The fibre laser 1 shown in FIG. 1 comprises a fibre 2 which is locatedin a resonator. The resonator is formed by an end mirror 3 as well as anoutput mirror 4. At the end mirror 3, pump radiation 6 is coupled intothe active core of the fibre 2 via a pumping source 5. The pumpingsource 5 may be one or more laser diodes, for example. Due to a suitablecoating, the end mirror 3 is transparent for pump radiation and highlyreflective for laser radiation stimulated in the fibre 2. With respectto its diameter or its numerical aperture, respectively, the active coreof the fibre is dimensioned such that a plurality of transverse modescan be generated by said stimulation. Due to the inherent properties ofthe fibre 2, mode mixing of the radiation formed in the resonatoroccurs. Said mode mixing is additionally enhanced by a layout of thefibre 2 in bends 7. For example, it is possible to wind the fibre 2around a core.

A laser beam 8 exits from the output mirror 4 during pumped operation.The wavelength and the cross section of said beam are determined by thelaser activity in the fibre 2 as well as by properties of the outputmirror 4 which shall be described hereinafter.

The resonator of FIG. 1 comprises separate end mirrors 3 and 4. It ispossible, however, to provide the end surfaces of the fibre 2 directlywith one or both of said mirrors. FIG. 2 schematically shows the outputmirror 4. As can be seen, the output mirror 4 comprises two zones, aninner zone 9 and an outer zone 10. The inner zone 9 of the output mirror4 transmits radiation at the laser wavelength. In contrast thereto, itreflects the pump radiation. The outer zone 10, however, is reflectiveat both the wavelength of the laser radiation and the wavelength of thepump radiation and prevents pump or laser radiation from exiting in thearea of the outer zone 10.

FIG. 3 shows an enlarged top view of the fibre 2 in the area of the endsurface 11. The end surface is provided directly with the output mirror4; the outer zone 10 and the inner zone 9 are illustrated by differentlyshaded areas. The inner zone 9 is noticeably smaller than the crosssection of the fibre core 12. Since only the inner zone 9 transmitsradiation at the laser wavelength, laser radiation 8 is coupled out fromthe output mirror 4 only there.

Since laser radiation which consists of a mixture of transverse modes isgenerated in the fibre 2, i.e. in its fibre core 12, as alreadyexplained, not only pump radiation, but also the aforementioned modemixture, is directed on the output mirror 4 from the fibre side of themirror. The zone 9, which transmits laser radiation only in a partialregion of the fibre core 12, thus causes a mode selection such that theradiation with regard to the basic mode predemoninantly exits at theoutput mirror 4. Radiation of higher transverse modes is reflected backinto the fibre 2 where, due to the mode-mixing properties of the fibre2, said radiation ultimately couples in again into the low transversemodes transmitted by the output mirror 4, after several round-trips, ifrequired.

FIG. 4 shows a further representation of how the output mirror 4 can beembodied. It is not provided on the end surface 11 of the fibre 2therein, but is embodied as a separate, spaced apart element, which iseasier to manufacture. Expansion of the radiation exiting from the fibre2 occurs between the end surface 11 and the output mirror 4 withintermediately arranged optics 13. Said expansion has an effectespecially on the laser radiation exiting from the fibre core 12.

In this case, the radiation of higher transverse modes diverges morestrongly than that of the basic mode (not shown in FIG. 4). The outputmirror 4 arranged following the optics 13 principally corresponds tothat represented in FIG. 2, i.e. it comprises an inner zone 9, whichtransmits radiation at the laser wavelength, and an outer zone 10surrounding the inner zone 9 and reflecting radiation back again to theend surface 11 of the fibre 2 both at the laser wavelength and at thewavelength of the pump radiation.

The laser radiation of all transverse modes stimulated in the fibre 2 isdirected onto the output mirror 4, with the aforementioned beamexpansion being amplified by the varying divergence of the differentmodes. Therefore, in some cases, the surface area of the inner zone 9 isgreater than the cross sectional area of the fibre core 12, withouteliminating the desired preference of the low or basic mode by the innerzone 9 during transmission. The desired mode-filtering property of theoutput mirror 4 is ensured in that the surface area of the inner zone ismuch smaller than the expanded radiation of the low modes to beselected, in particular the basic mode, said expanded radiation comingfrom the fibre core 12.

In a graph 14, FIG. 5 shows the intensity I of the laser beam 8 over thecross section in the x-direction. As can be seen, an almost steppedprofile, which is referred to as a so-called top hat profile, is formedwith symmetry to the center z. Of course, this stepped profile requiresthat not only the basic mode is transmitted (whose intensitydistribution, while also having symmetry to the center z, does not dropin a step-like fashion), but also that the inner zone 9 causes highermodes to be admixed during transmission, so that the superposition ofthe radiation of the individual modes as a whole results in the steppedprofile. The admixing of higher modes or the composition of the outputlaser beam 8 of radiation of several transverse modes, respectively,naturally also has an effect on the propagation characteristics of thelaser beam 8, i.e. on the angle of divergence of the radiation.Radiation components of higher modes diverge more strongly.

The design of the inner zone 9 in relation to the fibre core 12 allowsthe beam profile or the propagation characteristics, respectively, to bedesigned as desired. There is no restriction to symmetric intensitydistributions here, as shown in FIG. 5, but it is possible also toachieve an asymmetric intensity profile or an asymmetric propagationcharacteristic, respectively, which does not have symmetry to the centerz, by means of an off-axis position of the inner zone 9, relative to theaxis of the fibre core 12 or to the axis of radiation exiting therefrom,respectively.

This effect of the inner zone 9 is schematically illustrated in FIG. 6,which shows an intensity profile 15 corresponding to the basic mode. Theintensity profile 15 drops from a maximum to a 1/e²-proportion, over aradius r, starting from the center z. In multi-mode fibres, the radius ris clearly smaller than the fibre core radius a, which correspondsapproximately to the radius of the intensity distribution 14 ofmulti-mode radiation. By coupling-out radiation within the radius r,radiation of the basic mode is preferred and the emitted laser radiationin the laser beam 8 has a better beam quality than the pump radiation 6.The pump radiation 6 may be coupled in over a larger cross section atthe end mirror 3, which increases the maximum power that can be coupledin and thus the power of the fibre laser 1. The limitation due to thebrilliance of the pumping source 5 is thus eliminated. The intensity ofhigher modes has a slighter radial drop than that of the basic mode;thus, higher modes extend transversely over a greater radius. Therefore,use is made of a fibre 2 whose fibre core 12 has a greater radius thanthe radius r. The coupling-out radius is smaller than the radius a andpreferably greater than the radius r.

1-9. (canceled)
 10. A laser comprising a resonator which is limited byan end mirror and an output mirror and in which a fibre is arranged thatcomprises an active core and can be stimulated by pump radiation to havemulti-mode laser activity such that a plurality of transverse modesoccur in the resonator, wherein mode mixing occurs in the fibre, andwherein the output mirror has reflective properties for laser and pumpradiation varying such that the output mirror reflects pump radiation aswell as laser radiation that does not exit from the active core of thefibre and thus predominantly couples out low transverse modes.
 11. Thelaser of claim 10, wherein said fibre has a layout in loops or in bendsso as to promote mode mixing.
 12. The laser of claim 10, wherein thefibre active core has a D-shaped cross section.
 13. The laser of claim10, wherein said output mirror is exchangeable, thereby providing forswitching a wavelength of the laser or a diameter of a laser beamemitted from the laser.
 14. The laser of claim 10, wherein the outputmirror is exchangeable and comprises an inner zone and an outer zonesurrounding said inner zone.
 15. The laser of claim 14, wherein saidouter zone reflects laser radiation and pump radiation and said innerzone has a lower reflectivity for laser radiation than the outer zone.16. The laser of claim 14, wherein the inner zone is not coaxial to theradiation exiting from the active core.
 17. The laser of claim 14,wherein the inner zone is generally circular, having a smaller diameterthan the diameter of the active core.
 18. The laser of claim 10, furthercomprising beam-expanding optics arranged between an end of the fibreand the output mirror.
 19. A method of generating a laser beam from alaser, the laser comprising a resonator limited by an end mirror and anoutput mirror, a fibre comprising an active core and stimulated by pumpradiation to have multi-mode laser activity such that a plurality oftransverse modes occur in the resonator, wherein mode mixing occursproximate an end of the fibre, and wherein the output mirror has varyingreflective properties for laser and pump radiation such that the outputmirror reflects pump radiation and laser radiation not exiting from theactive core, thereby coupling out low transverse modes, the methodcomprising activating said laser.
 20. A method of manufacturing a laser,comprising: providing a pumping source; and positioning a fibre betweenan end mirror and an output mirror such that said pumping source willemit a light beam directed at said end mirror, wherein said fibercomprises an active core and can be stimulated by pump radiation to havea multi-mode laser activity such that a plurality of transverse modes,wherein mode mixing occurs in said fibre when said laser is activated;and wherein said output mirror has reflective properties for laser andpump radiation varying such that the output mirror reflects pumpradiation and laser radiation not exiting from said active core, therebycoupling out low transverse modes.
 21. The method of claim 20, whereinsaid fibre looped so as to promote mode mixing.
 22. The method of claim20, wherein said fibre is bent so as to promote mode mixing.
 23. Themethod of claim 20, wherein the output mirror is exchangeable andcomprises an inner zone and an outer zone, thereby providing forswitching a wavelength of a beam emitted by the laser or a diameter of abeam emitted by the laser.
 24. The method of claim 23, wherein saidinner zone is not coaxial to radiation emitted from the active core. 25.The method of claim 23, wherein the inner zone is generally circular andhaving a smaller diameter that a diameter of the active core.
 26. Themethod of claim 20, wherein the laser further comprises beam-expandingoptics arranged between an end of the fibre and the output mirror.